The danger of mobile phones is under fresh scrutiny after a study found radiation emissions can affect the body without heating up tissue.

The findings challenge fears that heating from mobile phone signals is their only potential threat to brain cells.

There is no evidence that mobile phones harm human health, but some limited research suggests that radiation from mobiles can speed up the growth of human tissue

In laboratory tests, scientists at Nottingham University, UK, have found that microwave emissions typical of mobile phones make a type of earthworm more fertile.

These results are very important and potentially far-reaching

Sir William Stewart

There is no suggestion that human fertility could be affected, but according to a report in New Scientist magazine, the results provide the first clear evidence that mobile phone radiation may have biological effects without warming tissues.

Scientists found the larvae of tiny nematode worms grew faster and became more fertile after they were exposed for a long time to weak microwave radiation, of the same strength and frequency as that emitted by mobile phones.

The proportion of worms that matured into egg-bearing adults was 28-40% higher.

By comparison, worms that had been exposed to mild heat suffered a 10% reduction in their growth rate, and virtually none developed into fertile adults.

Sir William Stewart, who chairs an expert group co-ordinating a new £7.4m research programme in the UK looking into the health risks of mobile phones, said: "These results are very important and potentially far-reaching.

Unusual behaviour

"Independent information is crucial and we need this quickly."

David de Pomerai, who led the research, said the findings were significant because even mild heating would normally make larval worms infertile as adults.

In previous experiments, De Pomerai's team studied worms that were genetically modified to generate a heat-stress protein when exposed to stresses other than heat.

Prolonged exposure to microwave radiation produced the stress protein, even though there appeared to be no noticeable heating.

In addition, the worms grew about 10% larger than worms not exposed to the radiation.

The experiments were inconclusive, however, because of the problem of measuring the temperature of such tiny worms.

The new research, on the other hand, ruled out any heating effect.

One theory to explain what is happening is that water molecules agitated by microwaves are attracted to water-seeking areas on the surface of proteins, influencing their ability to function.

But Mr De Pomerai said a mechanism still had to be established.

The latest results are unlikely to have an immediate impact on mobile phone safety advice.

Michael Clark from the National Radiological Protection Board, the
Government's radiation watchdog, said: "The guidelines can't be changed on the basis of one experiment.